The invention relates to the output stage employed in a monolithic silicon integrated circuit (IC) amplifier. One well-known form of output stage is disclosed and claimed in U.S. Pat. No. 3,974,456, which was issued to Ronald W. Russell and Kyle M. Black on Aug. 10, 1976. This patent, which is assigned to the assignee of the present application, teaches a pair of NPN output transistors biased by means of P-channel junction field effect transistor (JFET) devices. This circuit is exemplified in the LF156 which is available commercially from National Semiconductor Corporation and others.
Another commonly used IC amplifier output stage design which employs a complementary transistor is exemplified in the LM163, which is also commercially available from National Semiconductor Corporation and others. Here, an NPN transistor acts as an output terminal current source and hence provides the pull-up function. A PNP transistor acts as a current sink to provide the pull-down function. This configuration provides excellent performance but suffers an asymmetry in its current sinking capability. The PNP transistor is not able to sink as much current as an equivalent size NPN transistor can source. It also suffers to a small degree, at the higher frequencies, from an asymmetry in the gain versus frequency roll off. In general, IC PNP transistors have a much lower gain bandwidth than the NPN transistors. When the complementary device circuit is employed at the higher frequencies, the output will display a phase shift between the positive and negative signal swings. Finally, in junction isolated IC structures the PNP devices are commonly of vertical construction, particularly in those cases where the collector is dedicated to the IC substrate which is the most negative circuit potential. In such a case all of the current that flows in the PNP transistor is in the form of carriers that must flow in the IC substrate. Such carriers must flow across the substrate to a nearby contact thereby developing undesired potential drops.
To overcome some of the problems of the complementary device circuit a quasi-complementary circuit has been used. Here a pair of NPN output transistors are employed, but one of them, the current sink device, is driven by a PNP transistor to create a composite pair that acts as if it were a single PNP transistor. In this circuit the actual PNP transistor is usually of lateral construction and only has to handle low level signals. Thus, substrate currents are avoided, but the circuit is still slow so that a high frequency phase shift asymmetry is encountered. One version of such a circuit is found in the patent application of Harry J. Bittner Ser. No. 493,539, filed May 11, 1983, and assigned to the assignee of the present invention. Here a current mirror mirror is employed to drive the quasi-complementary output stage.
From the above, it is clear that NPN output transistors are preferred for use in the output stage of IC devices. However, the use of JFET biasing or quasi-complementary circuits have drawbacks that would be desirable to avoid.